Investigating polyol and acetic acid metabolism in wine related non-Saccharomyces yeasts

ENGLISH SUMMARY: Glycerol is the main polyol produced in Saccharomyces cerevisiae not only to counterbalance osmotic pressure but also to adjust redox balance. Incidentally, it may also contribute to the smooth mouthfeel of wine. Whereas glycerol is closely linked to acetic acid production in S. cerevisiae, this correlation is not as clear in non-Saccharomyces yeasts (particularly Torulaspora delbrueckii). Additional polyols - which function as stress protectants and could potentially influence wine mouthfeel - have been reported in wine but the producing yeasts were never isolated. Lachancea thermotolerans, Starmerella bacillaris and T. debrueckii have been recently described as producing other polyols in addition to glycerol with the latter producing the highest amounts. However, the enzyme assays used were limited to polyol detection in combination. Thus, the aim of this study was to optimize chromatography-based methods for the separation of polyols and to investigate the production of these compounds in non-Saccharomyces yeasts under a variety of environmental conditions. Gas Chromatography-Mass Spectrophotometry was successful for the separation of polyols but only in fermentation samples with no residual sugars. Since non-Saccharomyces yeasts do not ferment to completion, other methods are required for the individual detection of polyols in order to follow production throughout fermentation. Our data show that in addition to glycerol, three T. delbrueckii strains increasingly produced similar amounts of D-sorbitol, D-arabitol and D-mannitol throughout fermentation. Furthermore with the exception of glycerol, T. delbrueckii produced higher amounts of polyols in grape must when compared to synthetic must. Whereas glycerol is limited to NADH recycling, these additional polyols may increase the co-factor recycling pool in T. delbrueckii. Our data also show that D-sorbitol, D-mannitol and D-arabitol production was influenced by initial sugar concentration with the highest amounts detected for D-arabitol in T. delbrueckii. In contrast to D-arabitol which was produced at the highest amounts, D-mannitol and D-sorbitol were not induced by NaCl. It is possible that these compounds may have accumulated within the cell as a consequence of the osmotic gradients or mechanisms related to the prevention of ion toxicity as observed in literature. Polyol production was repressed in acetic acid media in this study and induced in ethanol supplemented media. The intake of acetic acid could have resulted in a change in redox balance and a reduced need for polyols as reported in literature. The presence of ethanol could have resulted in readjustment of polyol retention within the cell and release of polyols. Overall this study shows that non-Saccharomyces yeasts (particularly T. delbrueckii) are capable of polyol production. The amounts of polyols produced in some non-Saccharomyces yeasts may have a direct impact on wine but further investigations are required on this.